EZTOUCH to D0-06 PLC communication slow

I set up a simple system with a 6 inch EZTOUCH panel communicating via MODBUS over RS485 with 2 D0-06 PLCs from automation direct. The communication is working but is incredibly slow. Whe I hit a button on the panel it takes up to 5 seconds sometimes for the PLC to respond. I'm using 38.4 K baud so I would expect better response. Any idea what I'm doing wrong or is this normal? Thanks.
jim

Are you reading and writing random addresses in the PLCs? You'll speed things up if you put the data to be transferred in contiguous memory. I don't know the details of the EZTouch, but if it lets you configure alarms, use them sparingly.

Here's an example of why it pays off to make the addresses contiguous. Let's say that on one screen you are displaying the values in V2000 and V3000. Modbus has a limit of 256 bytes per message, so the EZTouch has to use two 'Read Register' messages to fetch the data. (Read 2 bytes of data starting at V2000, Read 2 bytes of data starting at V3000). If you were displaying the values in V2000 and V2001, the EZTouch could read them with a single message (Read 4 bytes of data starting at V2000).

Thanks Steve for the clarification. Unfortunately in this case it's not the problem as far as I can tell.I'm not writing/reading to/from Vmem locations yet . I'm only using discrete data ie. 1-C0 (for station 1) 2-C0 (for station 2). Any other thoughts.
jim

I found out that the Timeout Time for MODBUS on the panel was set to 30 tenths of a second. When I changed it to 1 tenth of a second the response improved signifigantly. What exactly does this timeout refer to and is it going to be a problem that I lowered it. Thanks for any advice.
Jim

I answered your post over at the AD forum. Here it is again for others to see.

It sounds like you are experiencing a timeout issue, or possibly you are just throwing lots of dtaa back and forth.

This could be caused by not using wire rated for RS-485 (we recommend Belden 9841), or possibly noise on the line (you did ground the shield at one end only, right?), or not having termination resistors.

About the only way to know for sure if it is noise is to put either an oscilloscope on it or use a communications analyzer.

Click to expand...

PS. From what you just said in this forum I can definitely say that it is a timeout issue. Lowering the timeout value doesn't fix the problem, it just masks it by not taking so long to retry the failed communications packet. Over time it will probably get worse if you don't dorrect it now.

I'm only driving 2 plcs about 3 feet apart and 2 feet from the panel. I didn't think cabling would be an issue at such a short distance? You still think it could be the problem?
jim

Quite possibly. If the impedence is off quite a bit then you could be having a reflection problem. In other words your signal isn't absorbed by the end of the cable run and reflects back upon itself causing the signal to turn into garbage.

While you can sometimes get away with not using termination resistors for such a short run, line impedence is a bit more important.

Actually impedence and capacitance take a roll as well as inductive coupling, but for the most part impedence is usually the culprit.

Get a piece of Belden 9841 and try it. Be sure to ground the shields at one end only. The EZ-Touch should be on the end of the run and it has built in termination resistors on pin 9 of the port. For the DL06, you will have to add a termination resistor of about 120 ohms.

A pinout for this application can be found in the AutomationDirect Technical Application Notes section of the Tech Support website.